1. Field of the Invention
The invention relates to a floating construction having a base, a tensioning member extending from the sea bed in the direction of the base, a connector at an upper end of the tensioning member and attachment means on the base for attaching to the connector.
2. Description of the Related Art
Tension leg platforms (TLP's) that are anchored to the sea bed via taut tendons or tension legs are generally known. For installation of the platform, which is often formed by a semi-submersible structure, the hull is towed to the installation site without its top structure attached. The hull of the semi-submersible structure is anchored to the sea bed via pre-installed tendons. The normal installation procedure of a TLP requires the platform to be connected to the pre-installed tension legs. For this, the TLP is towed to its final mooring position, after which the TLP is ballasted down to its lock-off off draft (the draft at which a connection with the tension legs is made). The upper ends of the tension legs are then aligned with connection sleeves of the TLP hull, after which the tension legs can be clamped to gain a permanent connection. Finally, the TLP may be de-ballasted such that the TLP gains its operating draft and the tendons are tensioned.
When the TLP is in lock-off draft, the hull needs to be positioned precisely in line with the tension leg connection points, before the final connection can be made. The installation procedure not only requires precision at this point, but since the stability of the TLP may also be inadequate due to its reduced water-surface penetrating area, it also requires methods for providing stability during installation.
There are several ways to make a TLP more stable during installation. For example, a wider hull base may be used to increase the stability of the TLP, or the deck may be installed offshore after the hull is permanently moored. Offshore installation can only be done in good weather and it is expensive and dangerous. Another method to increase the stability during installation relies on the use of specialized installation equipment such as installation support vessels or temporary buoyancy vessels. Since there are only a small number of vessels capable of providing the required stability, this method is also costly.
U.S. Pat. No. 7,044,685 describes a method for installing a TLP by connection to its tendons using pull-down lines to rapidly submerge the hull to installation draft while compensating for inherent hull instability during submergence. The system includes tensioning devices mounted on the TLP, usually one for each tendon and clamping connection sleeves on each corner of the hull for attaching to the upper end of the tendons. The TLP is submerged to lock-off draft by applying a tensioning force to the pull-down lines connected to the top of the tendons. Additionally, the hull may be ballasted to aid in submerging the TLP. The TLP is then positioned such that the upper ends of the tension legs can be fixed inside the connection sleeves.
The above mentioned installation procedure provides improved stability during installation, since it provides a downward force at the various connection points. In this way, it is not possible for the unstable hull to capsize, since this requires that one side of the hull rotates upward. The downward force provided by the tensioning devices prevents this from happening.
The known connection of the upper ends of the tendons to the hull causes relatively large bending forces that are exerted on the releasable connection of the sleeves. Furthermore, the known method of connection of the upper ends of the tendons to the hull has as a drawback that the tendons must be accurately aligned with the connection sleeves, which requires positioning of the hull by the installation support vessel. All the different tension lines must be reeled in at a correct position, before said connection can be made. This is a time consuming task and requires a large degree of precision. The use of remote operating vehicles (ROV) is required to check whether each tension leg is in a correct connection position.